1. Field of the Invention
The present invention relates to a method for controlling, in the processing of effluent with activated sludge, the amount of activated sludge to be returned from a precipitation tank to an aeration tank, in particular, a method for returning activated sludge of an amount matching the determined fluctuation of the organic material concentration in effluent. The present invention relates also to an activated-sludge processing apparatus suitably used in conducting the above method.
2. Description of the Related Art
Processing of effluent with activated sludge is based on a principle that the organic material in effluent is decomposed in an aeration tank by the microorganisms in activated sludge. Therefore, when the amount of organic matter flowing into the aeration tank is larger than the decomposability possessed by the microorganisms in activated sludge, the decomposition of the organic matter is incomplete and the effluent is discharged in a state of incomplete processing. Therefore, in designing an apparatus for effluent processing with activated sludge, it has heretofore been customary to allow an effluent-controlling tank to have a large size in order to make substantially constant the effluent load. Further, the amount of activated sludge to be returned to the aeration tank in order to maintain the sludge amount therein at a required level has been fixed depending upon the design conditions used; and, in many cases, the amount has not been varied so as to match the fluctuation of the organic matter load in effluent.
Meanwhile, JP-A-9-229923 discloses an apparatus for measuring the pollutant load in effluent by the use of a biosensor, used in processing of sewage or effluent. According to the literature, effluent is passed through a pre-processing part, i.e. a suspended matter-removing unit using a hollow fiber membrane module to remove the suspended matter in effluent (the suspended matter may clog the effluent pipe) to send the effluent stably to a biosensor; then, the suspended matter-removed effluent is introduced into a measurement unit such as a harmful matter measurement section, an organic matter load measurement section or a nitrification group load measurement section (either of these sections has a biosensor) to examine the loads of pollutant (organic matter and nitrification group) to the microorganisms in activated sludge, their proportions, and the adverse effect of harmful matter on the microorganisms in activated sludge; thereby, the effective control of sewage or effluent processing is made possible.
In the above conventional technique, a different sensor is required for each matter to be measured; further, a pretreatment (e.g. removal of suspended matter) is necessary; furthermore, the organic matter, etc. adhering to or contained in the suspended matter are not measured; therefore, the measurement of true organic matter load in effluent is impossible.